The present invention generally relates to protective circuits for electric motors, and more particularly to a protective circuit for an electric motor, designed to detect abnormal decrease in the rotation of the electric motor when the rotation of the electric motor decreases abnormally due to causes such as overload on the electric motor, and cut off the supply of current to the electric motor.
Generally, when the load on an electric motor becomes excessively large, including the case where the rotation of the motor is mechanically restricted, the rotation of the motor greatly decreases or the motor stops rotating. However, in such a case, a control circuit for the motor operates so as to supply a larger current to the motor and rotate the motor at a constant speed. Hence, if the current is continuously supplied to the motor, the motor will abnormally generate heat to result in the breakdown of the motor.
Accordingly, there was a conventional device for protecting the motor, wherein temperature detecting devices were provided on a main motor body and a heat generating part of a driving device. In this device, the temperature detecting devices detected the generation of heat at the main motor body and/or the driving device caused by the overload, and the current supply to the motor was cut off according to the detected outputs of the temperature detecting devices, in order to protect the motor. However, this conventional protective device was designed to detect an abnormal state by detecting the above generation of heat. Thus, a relatively long period of time was required before such an abnormal state could be detected, and there was a disadvantage in that the degree of protection with respect to the motor was insufficient and the motor could break down before the abnormal state is detected. In addition, in the conventional protective device, the temperature detecting devices had to be provided on both the main motor body and the driving device. Therefore, this resulted in an increased number of parts, and there was a further disadvantage in that the protective device could not be manufactured at low cost.